Free space optical (FSO) transmission is a type of information exchange using electromagnetic waves, or optical signals, to carry information. FSO transmission may require no propagation medium, such as fiber-optic cable or electrical wire, for successful information transmission. For example, electromagnetic signals carrying information in their amplitude, frequency, polarization, phase, etc. may be sent and received through space, from one point to another.
FSO receiving and/or transmitting devices may be referred to as telescopes. FIG. 1 is an illustration of a pair of conventional coaxial FSO telescopes 100 and 200. As used herein, the term “coaxial” includes the use of a common aperture/path for both transmitted and received signals. As shown in FIG. 1, coaxial telescope 100 may use a shared path 105 for both sending and receiving optical signals 101 and 102. Telescope 100 may include one or more transmitters 111 and receivers 112 optically coupled to a filter 120. Transmitter 111 may be any type of known optical transmitter capable of generating and encoding information onto electromagnetic signals; similarly, receiver 112 may be any type of known optical receiver capable of perceiving electromagnetic signals and decoding information from them.
Transmitted optical signals 101 and received optical signals 102 may at times overlap in space through shared path 105, such that telescope 100 may receive and transmit electromagnetic signals through a single, small aperture or other sending/receiving portal. Signals 101 and 102 conventionally use different wavelengths of light so that a filter 120 or other optical combiner/separator component may discriminate among received and transmitted optical signals 102/101. For example, filter 120 may reflect a particular wavelength of light used for receiving signals 102 and transmit a different wavelength of light used for transmitting signals 101.
As shown in FIG. 1, a transmitted signal 101 of a first frequency may pass through filter 120, while a received signal 102 of a second frequency may be reflected to a receiver 112 by filter 120. Thus, in order to differentiate between coaxial transmitted/received signals 101/102, receiver 112 and transmitter 101 may not be co-located. Similarly, in order to maintain the coaxial nature (overlap) of send/receive signals 101/102 without signal angular divergence and/or transverse offset (borosight error), positions and angles of receiver 112, transmitter 111, and filter 120 may each correspond and be maintained in relatively precise relative alignment.
Another coaxial telescope 200 may be positioned on the opposite end of path 105 and be substantially similar to the above-described telescope 100. The receiver 212 and transmitter 211 may be in opposite positions, if filter 220 has the same optical properties of filter 120. Alternately, filter 220 may have different optical properties and receiver 212 and transmitter 211 may be reversed based on which signals filter 220 reflects and transmits. In this way, signals 101 and 102 may be transmitted between telescopes 100 and 200 through a shared transmit/receive path 105 through free space.
FIG. 2 is an illustration of another conventional FSO coaxial telescope 300. As shown in FIG. 2, telescope 300 may possess several similar features of telescopes 100 and 200, including a transmitter 311, receiver 312, and filter 320. Transmitted signal 301 and received signal 302 may similarly share a transit path 305. Conventional coaxial telescope 300 may further include a secondary mirror 321 that reflects signal 301 to filter 320, where it is subsequently reflected into shared path 305. Mirror 321 may permit both receiver 312 and transmitter 311 to be co-located, thereby permitting greater linear integration and size reduction of telescope 300. Positions and angles of receiver 312, transmitter 311, filter 320, and mirror 321 may each correspond such that the coaxial nature of send/receive signals 301/302 may be maintained without significant signal angular divergence and/or transverse offset (borosight error).